"Last week, I determined that we had enough intelligence to take action and authorized an operation to get Osama bin Laden and bring him to justice," Obama said. "Today, at my direction, the United States launched a targeted operation against that compound in Abbottabad, Pakistan. A small team of Americans carried out the operation with extraordinary courage and capability. No Americans were harmed. They took care to avoid civilian casualties. After a fire fight, they killed Osama bin Laden and took custody of his body."

Capturing or killing the Al Qaeda leader had been his top priority in the war on terrorism, Obama said, and he called Sunday’s operation “the most significant achievement in our effort to date to defeat Al Qaeda.”

In a statement that perhaps would resonate most profoundly, he said simply, “Justice has been done.”

It was a stunning announcement in many respects, one that riveted late-night television viewers and drew tens of thousands of people to the White House after midnight – and to Times Square and Ground Zero as well – where they cheered, waved American flags, and sung the National Anthem.

It had become a cliché to say that “everything changed” after 911, but in many ways it was a transformational point in US history.

Americans got used to much stricter security measures at airports, the concrete barriers around the White House and other potentially vulnerable sites, color-coded threat levels, and the other attempted attacks on US targets – the “shoe bomber,” the “underwear bomber,” the failed or thwarted bomb attacks in New York, Portland, Oregon, and other locations.

The years since 911 also had seen the steady transformation of US armed forces away from the Cold War-era training, equipping, and disposition of military units to an increasing emphasis on counter insurgency (COIN). Coincidentally, the demise of Al Qaeda’s leader came just days after Obama had announced new leaders at the Pentagon and the CIA as well as the commander of US-led forces in Afghanistan.

Months and years will be spent detailing and learning the historical lessons of this period as they apply to US military and diplomatic approaches to threats around the world. His late Sunday statement may be seen as the most important of Obama’s first term as commander-in-chief.

Former president George Bush issued a statement of gratitude and congratulations. Analysts noted that it was exactly eight years to the day since Mr. Bush had given his “mission accomplished” speech aboard an aircraft carrier returning from the war in Iraq – words that turned out to have been more hopeful than accurate.

Anticipating the possibility of retaliatory attacks, officials say security will be tightened around the world. The State Department warned of "enhanced potential for anti-American violence" following bin Laden's death.

“There’s no doubt that al Qaeda will continue to pursue attacks against us,” Obama warned. “We must – and we will – remain vigilant at home and abroad.”

“As we do, we must also reaffirm that the United States is not – and never will be – at war with Islam,” he added. “I’ve made clear, just as President Bush did shortly after 9/11, that our war is not against Islam. Bin Laden was not a Muslim leader; he was a mass murderer of Muslims. Indeed, al Qaeda has slaughtered scores of Muslims in many countries, including our own. So his demise should be welcomed by all who believe in peace and human dignity.”

Beyond the major news in his announcement, Obama acknowledged the ongoing pain of those who lost loved ones on 911.

“We know that the worst images are those that were unseen to the world,” he said. “The empty seat at the dinner table. Children who were forced to grow up without their mother or their father. Parents who would never know the feeling of their child’s embrace. Nearly 3,000 citizens taken from us, leaving a gaping hole in our hearts.”

Families of those lost welcomed the presidential regard for their circumstances.

"This is important news for us, and for the world. It cannot ease our pain, or bring back our loved ones," Gordon Felt, president of Families of Flight 93, said in a statement. "It does bring a measure of comfort that the mastermind of the September 11th tragedy and the face of global terror can no longer spread his evil."